The production of hydrogen peroxide as such is well known. Hydrogen peroxide can be produced by various methods, e.g., by direct hydrogenation of oxygen or more commonly by the so-called anthraquinone autoxidation process (AO-process). The present invention particularly relates to the more industrially common AO-process.
Hydrogen peroxide is one of the most important inorganic chemicals to be produced worldwide. The world production of hydrogen peroxide grew to 3.2 million metric tons (100% H2O2) in 2009. Its industrial application includes textile, pulp and paper bleaching, paper recycling, organic synthesis (propylene oxide), the manufacture of inorganic chemicals and detergents, environmental and other applications. In the context of the present invention the industrial application pulp and paper bleaching, mining or environmental applications are of particular interest.
Hydrogen peroxide production is performed by a few chemical companies that produce it in large scale plants as an up to 50-70 percent concentrate in water (% by weight). Because of the highly oxidative characteristics of that level of concentration hydrogen peroxide usually is adjusted to a 50 percent concentration for safe handling and transport, and 70 percent concentrates normally are used only for transport over large distance due to cost reasons. For safety reasons the hydrogen peroxide product is normally diluted to at least 50% before application, but for most applications it will be applied in a concentration of below 15%, In order to minimize operations, the dilution to the effective concentration normally occurs within the application itself by adding the appropriate amount of a higher concentrated solution of usually not more that 50% hydrogen peroxide. Ultimately, hydrogen peroxide is used in various concentrations depending on the application, e.g., in a variety of applications hydrogen peroxide is used in a concentration of approximately 1-15 percent. Some particular examples of such hydrogen peroxide concentrations are, depending on the kind of industrial application: pulp bleaching 2-10%; waste water oxidation 1-5%; consumer products surface cleaning 1-8%. In other applications such as disinfection the hydrogen peroxide concentration may be higher, e.g. in aseptic packaging typical concentrations may be 35% or 25%.
Industrial synthesis of hydrogen peroxide is predominantly achieved by using the Riedel-Pfleiderer process (originally disclosed in U.S. Pat. Nos. 2,158,525 and 2,215,883). This well-known large scale cyclic production process of hydrogen peroxide makes use of the autoxidation of a 2-alkylanthrahydroquinone compound to the corresponding 2-alkylanthraquinone which results in the formation of hydrogen peroxide.
Thus, hydrogen peroxide is typically produced using a two-stage cyclical anthraquinone process (AO-process) comprising the hydrogenation of anthraquinone working solution in a catalytic reactor and the oxidation of the hydrogenated anthraquinone working solution by air in a multi-stage packed bed or sieve plate tower while simultaneously producing hydrogen peroxide in the organic stream, with the consecutive extraction of the hydrogen peroxide from the anthraquinone working solution by water in a multistage counter-current extraction column process. The organic solvent of choice is typically a mixture of two types of solvents, one being a good solvent of the quinone derivative (usually a mixture of aromatic compounds) and the other being a good solvent of the hydroxyquinone derivative (usually a long chain alcohol or cyclic ester). Next to said main AO-process steps, there may be other ancillary process steps involved, such like the separation of the hydrogenation catalyst from the working solution; the recovery and polish purification of the anthraquinone working solution, the accompanying solvents, and their recycle to the hydrogenator; and the recovery, polish purification and stabilization of the hydrogen peroxide product.
This AO-process utilizes alkylanthraquinone compounds, such as 2-ethylanthraquinone, 2-amylanthraquinone, and their 5,6,7,8-tetrahydro derivatives as the working compounds dissolved in a suitable organic solvent or mixture of organic solvents. These solutions of alkylanthraquinones are referred to as working solutions. In the first stage of the anthraquinone process (hydrogenation step), the working solution is subjected to hydrogenation in order to reduce the working compounds to their hydrogenated form, the alkylhydroanthraquinones. The hydrogenation of the working compounds is accomplished by mixing hydrogen gas with the working solution and contacting the resulting solution with an appropriate hydrogenation catalyst. In the second stage of the two-stage AO-process (oxidation step), the hydrogenated working compounds, i.e., the alkylhydroanthraquinones, are oxidized using oxygen, air, or a suitable oxygen containing compound in order to produce hydrogen peroxide and restore the working compound to its original form. The hydrogen peroxide produced in the oxidation step is then removed from the working solution, typically by extraction with water, and the remaining working solution containing the alkylanthraquinones is recycled to the hydrogenation step to again commence the process. The hydrogenation step may be carried out in the presence of a fluid-bed catalyst or a fixed-bed catalyst. Either method is known to have its particular advantages and disadvantages.
In a fluid-bed hydrogenation reactor, good contact between the three phases therein is obtained and thus the productivity and selectivity are generally high. However, the catalyst particles can be broken down by abrasion and can block the filters needed to separate the suspended catalyst and the hydrogenated working solution. This kind of reactor is also subject to back mixing. So, the use of suspended catalyst frequently requires the use of a larger hydrogenation reactor and expensive filtration sector to obtain a fully hydrogenated form.
In the fixed-bed hydrogenation reactor the catalyst does not abrade as much as the fluid-bed reactor and, if operated in a concurrent flow, does not result in back-mixing. But the reaction rate of a fixed-bed hydrogenation reactor is limited by the relatively slow rate of dissolution of hydrogen from the gas phase into the working solution, and also by the proportionally lower Pd surface per unit weight of a fixed bed versus a fluid bed catalyst. Therefore, to dissolve the required quantity of hydrogen necessary to thoroughly reduce all of the working compounds, the working solution has normally to be recycled several times. Thus, a very large recycle stream and a correspondingly large hydrogenation reactor are required, and thus adding to the capital costs of the process. In addition, the recycling of the hydrogenated solution results in over-hydrogenation of the working compounds so that they are ineffective in the overall process.
A special kind of fixed-bed reactors are the so-called trickle-bed reactors which are generally known in the literature (see e. g. NG K. M. and CHU C. F. Chemical Engineering Progress, 1987, 83 (11), p. 55-63). Although the trickle-bed reactors are primarily used in the petroleum industry for hydrocracking, hydrodesulfurization, and hydrodenitrogenation, and in the petrochemical industry for hydrogenation and oxidation of organic compounds, nevertheless, the trickle-bed hydrogenation reactor is also found in some versions of the AO-process for the manufacture of hydrogen peroxide. The term trickle-bed is used here to mean a reactor in which a liquid phase and a gaseous phase flow con-currently downward through a fixed bed of catalyst particles while the reaction takes place. Current practice in operating the trickle-bed reactor still relies mainly on empirical correlations and obviously parameters such like pressure drop, dispersion coefficients, and heat and mass transfer coefficients depend on both, gas and liquid flow rates. From the literature it is also known to operate trickle-bed reactors under different flow patterns such like “trickling”, “pulsing”, “spray”, “bubble” and “dispersed bubble”. One of the major problems in the use of the trickle-bed, especially in the trickle-flow regime, is the possibility of channeling in the fixed-bed hydrogenation reactor.
In Chemical Abstracts no. 19167 f-h, volume 55 (Japanese patent no. 60 4121) the manufacture of hydrogen peroxide is described, in which process a mixture of an alkylanthraquinone solution with great excess of hydrogen or hydrogen containing gas is foamed by passing this mixture through a porous diffuser in the upper part of a column containing a granular hydrogenation catalyst. The foamed mixture is then passed rapidly through the catalyst layer to hydrogenate. At the bottom of the column a bed consisting of glass wool, rock wool, a metal screen, or a filter cloth is placed for defoaming, and the defoamed working solution is withdrawn from the bottom and the great excess of separated gas is recycled to the column. Although by this process the possibility of channeling in a fixed-bed hydrogenation reactor in the trickle-flow regime is eliminated, according to the reference made to the Japanese patent in U.S. Pat. No. 4,428,922 (column 1, lines 55 to 68) there are still some drawbacks left, like high pressure-drop, the increase of energy consumption related thereto, a large recycle stream of hydrogen back through the hydrogenation reactor, and the use of additional equipment required for foaming and defoaming the working solution. In addition the through-put or productivity of the hydrogenation reactor is drastically reduced because of the large volume occupied by the excess hydrogen gas.
The WO 99/40024 aims to overcome the disadvantages of the above described processes and provides a simplified and advantageous process for the manufacture of hydrogen peroxide by the means of an AO-process using a fixed bed of catalyst particles in the hydrogenation step with high productivities.
This is achieved by a process for the manufacture of hydrogen peroxide by the AO-process comprising the alternate steps of hydrogenation and oxidation of a working solution containing at least one alkylanthraquinone dissolved in at least one organic solvent and extracting the hydrogen peroxide formed in the oxidation step, in which process the hydrogenation step is carried out in a hydrogenation reactor containing a fixed bed of the hydrogenation catalyst particles by feeding a concurrent flow of the working solution and a hydrogenating gas at the top of the reactor and by adjusting the ratio of liquid and gas feed-flows and the pressure of the hydrogenating gas to provide a self-foaming mixture of the working solution and the hydrogenating gas in the absence of any device or diffuser or spray nozzle for forming the foam, and by passing the foaming mixture downwards through the fixed bed of catalyst particles. The process of the WO 99/40024 may be carried out optionally batch-wise or in a continuous manner. In addition, the hydrogenation step of the process may be carried out optionally under a pulsing foam flow regime of the working solution through the fixed-bed catalyst. According to WO 99/40024 it is possible to carry out the hydrogenation step of the AO-process directly in a conventional hydrogenation reactor of trickle-bed type under foaming of the working solution and the hydrogenating gas without any additional special equipment for the generation of the foam. Thus the working solution and the hydrogenating gas can directly be fed into the hydrogenation reactor just by means of a conventional inlet pipe and the foaming is then achieved by adjusting the liquid and gas feed-flows and pressure of the hydrogenating gas. In addition there is no need to provide any special equipment at the bottom of the hydrogenation reactor for defoaming. The hydrogenation reactor containing the stationary trickle-bed employed in this is of conventional type and may take all forms and sizes generally encountered for the production of hydrogenation reactors of this type. Preferably the hydrogenation reactor is a tubular reactor (column).
According to WO 99/40024 there are several advantages related to the use of a foaming working solution in the hydrogenation step of the AO-process for the manufacture of hydrogen peroxide. The gas/liquid mass transfer is significantly increased because of the nature of the foam which consists of a dense dispersion of the hydrogenating gas in the liquid working solution. A good distribution of the working solution and the hydrogenating gas is maintained over the total length of the hydrogenation reactor. Consequently, for example the liquid surface velocity of a foaming working solution necessary to achieve good contact efficiency is approximately up to 2 to 3 times lower than that required for a non-foaming working solution. Thus, for foaming of the mixture of working solution and hydrogenating gas, the conditions in the hydrogenation reactor concerning the liquid and gas feed-flows and the pressure of the hydrogenating gas are set to provide a significant interaction of liquid (L) and hydrogenating gas (G); under these conditions the foam flow regime may be located between the trickle-flow and the pulsing flow regime. Accordingly, in the process of the invention the pressure of the hydrogenating gas is in the range of 1.1 to 15 bars (absolute), preferably in the range of 1.8 to 5 bars (absolute). The input superficial velocity of the hydrogen is usually at least 2.5 cm/s, preferably at least 3 cm/s. The input superficial velocity of the hydrogen is generally not higher than 25 cm/s, preferably not higher than 10 cm/s. The input superficial velocity of the liquid into the reactor is generally at least 0.25 cm/s, preferably at least 0.3 cm/s. The input superficial velocity of the liquid is generally not higher than 2.5 cm/s, preferably not higher than 1.5 cm/s, and more preferably, not higher than 1 cm/s.
The before described AO-processes based on the original Riedel-Pfleiderer concept are designed for the industrial large-scale and even up to mega-scale production of hydrogen peroxide. Thus, conventional hydrogen peroxide production processes are normally carried out in large- to mega-scale hydrogen peroxide production plants with production capacities of about 40,000 to 330,000 (metric) tons per annum of hydrogen peroxide per year. Thus, currently there are plants in industrial operation with a production capacity of e.g., 40 to 50 ktpa (kilo tons per annum) at the low end, with a capacity of up to 160 ktpa, and the world largest mega-plants provide a capacity of 230 ktpa (Antwerp) and 330 ktpa (Thailand). In these processes, normally the production capacity in case of fixed beds is limited to 50 ktpa and usually plants with production capacities above 50 ktpa are operated with fluid-bed reactors.
These conventional AO-processes and respective production plants are complicated and require many and large installments of equipment, a number of competent staff for maintenance of the equipment and operation of the main and ancillary process steps, and special safeguards to handle the resulting hydrogen peroxide in its usually high concentrations of 40 percent, and the further concentration to 50 to 70 percent. Hence, much management attention and frequent maintenance is required. In addition to the complexity of such large- to mega scale production processes, it is noted that a substantial part of the produced hydrogen peroxide needs to be transported, e.g., by train or truck, to be used by customers in their own industrial applications. Such transports by train and truck need special precautions in view of related safety and security issues.
On the other hand a variety of the customers' industrial applications of hydrogen peroxide do not require highly concentrated hydrogen peroxide solutions for their applications, and therefore, as already explained above, the hydrogen peroxide solutions which were concentrated for the purpose of an economic transportation, usually to a hydrogen peroxide concentration of about 50 percent, are only used in a lower concentration of e.g., 1 to 15 percent at the customer site for its specific local application, e.g., particularly for the use in the pulp and paper industry or the textile industry, or for use in the mining industry or for environmental applications.
Furthermore, the current large scale hydrogen peroxide AO-processes according to the Riedel-Pfleiderer concept typically are highly capital- and energy-intensive processes, and the costs associated with them are passed on to low-volume end users. These end users would benefit from methods for producing hydrogen peroxide more economically without the concomitant capital costs and handling problems associated with current production schemes in smaller local plant environments close to the end user's site.
The U.S. Pat. No. 5,662,878 (issued Sep. 2, 1997 and assigned to the University of Chicago) already discusses the need of a process that would allow effective hydrogen peroxide production in small plant environments at a “host” industrial site. Briefly, the U.S. Pat. No. 5,662,878 describes a method for producing hydrogen peroxide comprising supplying an anthraquinone-containing solution; subjecting the solution to hydrogen to hydrogenate the anthraquinone; mixing air with the solution containing hydrogenated anthraquinone to oxidize the solution; contacting the oxidized solution with a hydrophilic membrane to produce a permeate; and recovering hydrogen peroxide from the permeate. The proposed method for producing hydrogen peroxide claims as a feature the utilization of membrane technologies to isolate hydrogen peroxide from the process reaction liquid. The teaching of U.S. Pat. No. 5,662,878 focuses on the utilization of the membrane technology for producing hydrogen peroxide that is virtually free of organics, and the ability to retain expensive organic solvents in reaction liquors for reuse.
According to the U.S. Pat. No. 5,662,878 the Riedel-Pfleiderer AO-processes are considered unsuitable for small scale production of about scale production and medium scale production. This is because the packed tower used for oxidation, and the column for hydrogen peroxide extraction are very large and do not easily scale up or down for modularity and operational flexibility. Also, typical extractors are multi-stage, very large in volume and are deemed difficult to scale down and to tend being highly unstable, and thus requiring a high degree of operational control.
Although one might assume that the AO-process may be performed on small-to medium-scale so as to merely satisfy local demand, in the state of the art it is still deemed that such processes require the use of many pieces of equipment, much management attention, and frequent maintenance, and that they are difficult to scale down and difficult to make such processes profitable. But, despite the proposed process according to the U.S. Pat. No. 5,662,878 using a membrane technology, the industrial production of hydrogen peroxide still relies on large-scale production facilities and related process optimizations. Thus, no small scale production facility (500-5,000 metric tons per year) or medium scale production facility (5,000-20,000 metric tons per year) is operated up to now. It appears that industry either ignored the industrial potential of small to medium scale hydrogen peroxide production facilities or assumed technical and/or economical hurdles to apply such small to medium scale methods for producing hydrogen peroxide, as compared to the well-established large scale industrial production and available logistics to ship hydrogen peroxide, all despite the required hazardous concentrating by distillation and final concentration of the hydrogen peroxide for the purpose of shipping and finally required dilution for use at customer site.
Therefore, even today a very high need exists in the art to produce hydrogen peroxide without the concomitant capital costs and handling problems associated with current large-scale to mega-scale production schemes, and to develop new processes that would allow effective hydrogen peroxide production in small to medium size plant environments, particularly on a customer industrial site, on low-volume end users' sites or other suitable “host” industrial sites. Furthermore, these new small to medium hydrogen peroxide processes (“mini-AO processes”) should be as modular as possible with the ability for quick start-up, shut-down and turnaround, while also accommodating variability in production rates, and as simple and robust as possible to allow for an end user friendly plant which stably runs in continuous operation with a minimum need of local (e.g., on customer site) technical and/or physical intervention.
Hence, particularly in view of the economic significance of hydrogen peroxide, there is still a clear desire for small- to medium-scale hydrogen peroxide production plants which can produce aqueous hydrogen peroxide solutions for local use by applying the well-established AO-process technology according to the Riedel-Pfleiderer process, but which are also more cost effective manufacturing processes of hydrogen peroxide.